CA 15-3 is a blood tumor marker that measures the MUC1 glycoprotein often shed by breast cancer cells. It’s used mainly to monitor treatment response and detect recurrence in people with known breast cancer, not as a routine screening test. Levels can rise with advanced disease but may also be elevated in benign breast conditions and other cancers, so results require clinical correlation and imaging.

The typical normal range for CA 15‑3 is generally less than about 30–35 U/mL, though exact cutoffs vary by laboratory and assay. Mild elevations can occur with benign conditions; higher values may prompt further investigation in the context of symptoms or known breast cancer. CA 15‑3 is used primarily for monitoring treatment or disease progression, not as a standalone diagnostic test.

CA 15-3 has limited sensitivity and specificity: it’s not recommended for breast cancer screening or sole diagnosis. It’s most useful for monitoring known metastatic breast cancer—serial trends can suggest response or progression. Levels can be elevated for noncancerous conditions and may be normal despite recurrence, so results should be interpreted alongside clinical assessment and imaging, not used alone for treatment decisions.

CA 15‑3 and CA‑125 are different tumor markers: CA 15‑3 (MUC1) is mainly used to monitor breast cancer treatment response and detect recurrence, not for screening. CA‑125 (MUC16) is primarily used in ovarian cancer monitoring and risk assessment, though it can be raised by many benign gynecologic and non‑gynecologic conditions. Neither is sufficiently sensitive or specific alone for general population screening.

Common breast cancer markers include CA 15-3, CA 27.29 and CEA. CA 15-3 is typically considered normal below about 30 U/mL (lab cutoffs vary). CA 27.29 is often reported normal below about 38 U/mL. CEA is usually normal under 5 ng/mL in non‑smokers (smokers may have higher cutoffs). Results must be interpreted with imaging and clinical findings.

The earliest hint of metastatic breast cancer is often new, unexplained persistent symptoms—commonly bone pain (back, hips, ribs), a persistent cough or shortness of breath, new headaches or neurological changes, jaundice or upper abdominal pain, or unexpected weight loss and fatigue. Any new, ongoing symptoms after a breast cancer diagnosis should be reported promptly for evaluation and imaging.